1. Field of the Invention
The invention relates to the field of dynamic magnetic information storage or retrieval. More particularly, the invention relates to the field of automatic control of a recorder mechanism. In still greater particularity, the invention relates to track identification. By way of further characterization, but not by way of limitation thereto, the invention is a method for servo track identification which uses a track identification field in the servo pattern.
2. Description of the Related Art
Magnetic tape recording has been utilized for many years to record voice and data information. For information storage and retrieval, magnetic tape has proven especially reliable, cost efficient and easy to use. In an effort to make magnetic tape even more useful and cost effective, there have been attempts to store more information per given width and length of tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, this increase in the number of data tracks results in those tracks being more densely packed onto the tape. As the data tracks are more closely spaced, precise positioning of the tape with respect to the tape head becomes more critical as errors may be more easily introduced into the reading or writing of data. The tape--tape head positioning may be affected by variations in the tape or tape head, tape movement caused by air flow, temperature, humidity, tape shrinkage, and other factors, especially at the outside edges of the tape.
In order to increase data track accuracy, servo tracks have been employed to provide a reference point to maintain correct positioning of the tape with respect to the tape head. One or more servo tracks may be used depending upon the number of data tracks which are placed upon the tape. The sensed signal from the servo track is fed to a control system which moves the head and keeps the servo signal at nominal magnitude. The nominal signal occurs when the servo read gap is located in a certain position relative to the servo track.
Referring to FIG. 1, a one-half inch wide length of magnetic tape 11 may contain up to 288 or more data tracks on multiple data stripes 12. A thin film magnetic read head is shown in upper position 13 and lower position 14 to read data from data tracks 12. If a tape read head has sixteen elements and, with movement of the head to multiple positions, each element can read nine tracks, then that magnetic read head could read 144 tracks. In order to read more tracks, such as 288 in the desired configuration, two data bands 15 and 16 are utilized. The tape head is movable to nine tracking positions in each of upper position 13 and lower position 14. That is, with the tape head in position 13 it can read 144 tracks in data band 15 and in position 14 it can read 144 tracks in data band 16. With dual data bands 15 and 16 and multiple head positions within those bands, tape head positioning is critical.
In order to achieve accurate multiple head positions it may be desirable to include up to five or more servo stripes 17. Servo stripes 17 may utilize various patterns or frequency regions to allow precise tape to tape head positioning in multiple positions. This allows a data read head to more accurately read data from data stripes 12. Referring to FIG. 2, servo stripes 17 are shown in greater detail. As is disclosed in copending patent application entitled TAPE SERVO PATTERN WITH ENHANCED SYNCHRONIZATION PROPERTIES (attorney docket no. 96-010-TAP) filed on the same date as this application and hereby incorporated by reference, a first frequency signal 19 is written across the width of a frame 18 in each servo stripe 17. As is known in the art, a measurably different frequency such as an erase frequency is written over first frequency signal 19 in a predetermined pattern such as the checkerboard patterns in regions 21 and 22. The horizontal sides of twelve rectangles (20 and 23) in each stripe 17 are substantially parallel to the direction of movement of tape length 11. The six rectangles (12 sides) in each region 21 and 22 define five horizontal interfaces (servo tracks) 24 between frequency signal 19 and rectangles 20, 23 as the outside interfaces 25 along the top and bottom of each stripe 17 are ignored. In the preferred embodiment, rectangles 20 are shown on the left side of areas 21 and 22 and rectangles 23 are shown on the right side of areas 21 and 22. A servo read element 26 in a tape read head is precisely aligned along interface 24 to read the signal frequency along interfaces 24. That is, dotted line representing interface 24 along the horizontal sides of rectangles 20, 23 passes through the center of servo read element 26. If the servo pattern on the tape moves right to left, then servo read element 26 will alternate between reading frequency 19 across the full width of servo read element 26 between areas 21 and 22 and reading frequency 19 across one half of servo read element 26 and an erase frequency from rectangles 20, 23 across the other half of the width of servo read element 26. Thus, if tape 11 moves as shown in FIG. 2, servo read element 26 will first sense rectangle 20 above track 24 and then sense rectangle 23 below track 24 in each of regions 21 and 22.
As is known in the art, the servo control system in a tape drive determines the position error signal by using the ratio of the difference between the signal amplitude sensed during the first (left) half of patterns 21 or 22 and the signal amplitude sensed during the second (right) half of patterns 21 or 22 divided by the sum of the signal amplitude sensed during the first half of patterns 21 or 22 and the signal amplitude sensed during the second half of patterns 21 or 22 to stay on track. For a head position precisely on track in checkerboard pattern areas 21 or 22 shown in FIG. 2 the ratio will be zero because the signal during each half of the pattern will be the same. If servo read element 26 is above track 24, the polarity of the position error signal will be positive because more of rectangle 20 above track 24 and less of rectangle 23 below track 24 will be read. In response, the track servo will move the head (including servo read element 26) down until the ratio is zero and servo read element 26 is precisely on track 24. Conversely, if servo read element 26 is below track 24, the polarity of the position error signal will be negative because more of rectangle 23 below track 24 and less of rectangle 20 above track 24 will be read. In response, the track servo will move the head (including servo read element 26) up until the ratio is zero and servo read element 26 is precisely on track 24. In this way the tape controller can sense the position of the tape 11 with respect to the servo read element 26 and move the tape head to keep the head servo read element 26 aligned with the servo track along line 24. This alignment ensures precise reading of a data track in data stripes 12 by the data read head (not shown).
While the above described system is used to keep servo read element 26 (and in turn the read head) precisely on a track, the tape controller system does not know whether servo read element 26 is on the right track. As is known in the art, an optical sensor may be used to approximately position the tape head with respect to the tape. However, when precise positioning is required to position a read gap over a data track in data stripe 12, an optical sensor is not accurate enough. That is, with the expected range of tape motion due to guiding being significantly wider than track pitch, it is not possible to insure that track following will start on the desired track. This could result in the wrong track being read. It would be desirable to have a system in which the servo control circuitry could reliably determine upon which track 24 servo read element 26 is located.
A prior art solution to tape positioning is to have sufficient information recorded in the data tracks to permit proper identification of the track prior to starting a read or write operation. This approach requires the tape cartridge to be prerecorded at the factory to insure that all tracks had proper identification before being used in the field. Prewriting all tracks with sufficient information to properly identify each track adds to the cost of each cartridge. In addition, using data track space for identification information affects capacity because the amount of available space on a data track for actual storage of data is reduced.